Cylinder blocks (engine blocks), e.g., of internal combustion engines, are typically manufactured as a cast component. The inner surfaces of cylinders are directly manufactured in the casting process and usually do not fulfill (satisfy) geometric requirements and/or the requirements, with respect to the surface properties, placed on a cylinder bore, on or in which a piston will slide during operation, e.g., of the internal combustion engine.
For this reason, either cylinder liners, whose inner surfaces already correspond to the requirements or are appropriately processed after insertion, are inserted into the cylinders, or coatings are applied onto the inner surfaces of the cylinders. Such coatings may be applied, for example, using a thermally-sprayed coating method.
Known methods for applying thermal spray coatings include, for example, the Plasma Transferred Wire Arc method (“PTWA” method), the Electric Arc Wire Spray method (“EAWS” method) and the GROB® Thermal Spraying method (“GTS” method, which comprises the Rotating Single Wire (“RSW”) method and the Atmospheric Plasma Spray “APS2” method). In the latter, one end of an automatically-supplied wire forms an anode; a plasma beam emanating from a cathode impinges on the end of the wire thereby causing the end of the wire to melt. The melted wire material (e.g., iron (Fe) or an iron-containing metal material) is then atomized by a transport gas mixture and accelerated toward the cylinder bore (e.g., an aluminum alloy structure) to be coated. The metal particles entrained in the transport gas mixture are thus applied onto the cylinder bore (with deformation of the particles) and form the coating that optionally may be post-processed, e.g., by honing or by alternative finish-processing methods.
In series production using such thermally sprayed coatings, as well as in the development thereof, quality control is highly important. The quality of a sprayed coating has been determined in the past by measuring a set of coating properties that are analyzed in order to evaluate the quality of the sprayed coating. For this purpose, depending on the requirements of the evaluation method, polished specimens are produced in three spatial planes, which are subsequently evaluated using an optical (light) microscope. In most cases, the optical (light) microscope is equipped with a camera that captures (photographs) individual views as image data. These images (image data) have been analyzed using known image-processing programs in order to perform the analysis and evaluation of the coating properties.
Two important properties for evaluating the quality of a thermally applied coating, such as a coating applied according to the GTS method, are the proportion or ratio of pores (pore proportion) and the proportion or ratio of oxides (oxide proportion) in the coating.
Pores are cavities in a material and/or are recesses or openings with respect to the surface of a material. A differentiation is to be made between micro-, meso-, and macro-pores. In the present disclosure, the pore proportion of micropores (e.g., pores in the range of 1-5 microns) is preferably determined. The cavities/pores emerge as recesses after the removal of one or more layers that lie on top of the cavities/pores. The pore proportion therefore determines, inter alia, the oil retention volume of the surface of the cylinder bore. The amount of oil that can be retained on the surface of the cylinder bore is relevant to (contributes to determining) the friction properties of the surface.
In the present disclosure, the oxides are usually metal oxides, which result when a metal (e.g., iron (Fe) in the thermally-applied coating material) reacts with oxygen or another oxidizing agent. The metal oxides are also relevant, similar to graphite, to (contribute to determining) the friction properties of the surface.
Pore- and oxide-proportions within a region having homogeneous layer properties, i.e., a region in which the coating has been applied with nearly constant process parameters and environmental parameters, are preferably analyzed. In the case of a coated cylinder bore, a region of the coating that lies on essentially the same circumference circle about (surrounding, encircling) the longitudinal axis of the cylinder is essentially homogeneous. Depending on the manufacturing technology, differences in the coating can arise, in particular at different longitudinal positions of a cylinder.